In the manufacture of integrated circuits, wafer surface planarity is of extreme importance. Photolithographic processes are typically pushed close to the limit of resolution in order to create maximum circuit density. For 16-megabit dynamic random access memories, minimum critical dimensions, such as wordline and bitline width, will initially be in the 0.5.mu.-0.7.mu. range. Since these geometries are photolithographically produced, it is essential that the wafer surface be highly planar so that the electromagnetic radiation used to create a mask may be accurately focused at a single level, thus resulting in precise imaging over the entire surface of the wafer. Were the wafer surface not sufficiently planar, the resulting structures would be poorly defined, with the circuit being either nonfunctional or, at best, endowed with less than optimum performance.
In order to achieve the degree of planarity required to produce ultra high density integrated circuits, chemical-Micron mechanical planarization processes are being employed with increasing frequency. In general, chemical-mechanical planarization (CMP) processes involve pressing a semiconductor wafer against a moving polishing surface that is wetted with a chemically reactive, abrasive slurry. Slurries are usually either basic or acidic and generally contain alumina or silica particles. The polishing surface is typically a planar pad made of a relatively soft, porous material such as blown polyurethane. The pad is usually mounted on a planar platen.
FIG. 1 depicts a conventional rotational CMP apparatus. The apparatus comprises a wafer carrier 11 for holding a semiconductor wafer 12. A soft, resilient pad 13 is typically placed between the wafer carrier 11 and the wafer 12, and the wafer is generally held against the resilient pad by a partial vacuum. The wafer carrier 11 is designed to be continuously rotated by a drive motor 14. In addition, the wafer carrier 11 is also designed for transverse movement as indicated by the double headed arrow 15. The rotational and transverse movement is intended to reduce variability in material removal rates over the surface of the wafer 12. The apparatus further comprises a rotating platen 16, on which is mounted a polishing pad 17. The platen 16 is relatively large in comparison to the wafer 12, so that during the CMP process, the wafer 12 may be moved across the surface of the polishing pad 17 by the wafer carrier 11. A polishing slurry containing chemically-reactive solution, in which are suspended abrasive particles, is deposited through a supply tube 18 onto the surface of the polishing pad 17.
FIG. 2 illustrates the basic principles of the conventional rotational CMP process. The polishing pad 17 is rotated at an angular velocity of W.sub.P radians per second (rads./sec.) about axis 0. The wafer to be planarized 12 is rotated at an angular velocity of W.sub.w rads/sec., typically in the same rotational sense as the pad. It is easily understood that the linear speed (L) of the polishing pad, in centimeters/sec., at any given radius (r), in centimeters from axis 0, will be equal to W.sub.p r. Experience has demonstrated that the rate of removal of material from the wafer surface is related to the speed with which the pad surface makes contact with the wafer surface.
There are a number of disadvantages related to the conventional CMP process:
(1) From the foregoing discussion, it should be evident that achieving wafer planarity using conventional rotational CMP is no trivial task. The wafer must be carefully and continuously monitored during the material removal process.
(2) Conventional CMP equipment is complex, massive, and must be built to low-tolerance specifications. The conventional CMP method calls for two rotating surfaces to be brought into accurate planar contact with one another. Moreover, one of those rotating surfaces must be transversely moveable. Some of the massiveness is attributable to the fact that, as the diameter of the platen increases with respect to the diameter of the wafer being planarized, uniformity of removal rate can be maximized by utilizing the of the platen near the edge. In addition, if water cooling of the wafer carrier is implemented, the flow of water into the carrier requires the use of rotating seals. Such precision complexity ensures relatively high equipment costs.
(3) Maintaining temperature within a narrow range at the wafer surface is difficult because of the time lag inherent in the transfer of heat between the cooling water, the massive platen, the wafer carrier, and the wafer.
(4) Dishing occurs when material regions of two different hardnesses are present on the wafer surface. For example, aluminum lines imbedded in a silicon dioxide layer become recessed (dished) when subjected to conventional CMP. This problem is generally attributed to the high wafer-to-pad pressures involved, which cause distortion of the polishing pad. Pressures exerted on the wafer by the polishing pad are typically within a range of 0.3.times.10.sup.5 to 1.3.times.10.sup.5 pascals (approx 4 to 20 pounds per square inch).
(5) Conventional CMP equipment requires a continuous feed of slurry, which can be either discarded after being flung off the spinning pad and wafer, or captured and recirculated. In order to maintain the abrasive particles suspended in the liquid, slurry must be continually agitated.
(6) The polishing pads used in conventional CMP processes must be periodically reconditioned, due to the packing of abrasive particles into the pores thereof (a phenomenon referred to as "glazing").